The water-pressure transfer printing method has been known as a means for forming printed patterns, by way of transfer printing, on a formed body having irregular three-dimensional surfaces. To print on a curved surface by water-pressure transfer printing, a transfer printing film comprising a water-soluble or water-swelling film with a printing layer provided on top that constitutes pictures that have been activated with an active agent, is made to float on a water surface with the printing side facing up, and then the transfer target to transfer the pictures to is pressed on it from above and water pressure is utilized to transfer the images on the printing layer to the outer peripheral surface of the transfer target. In general, water-pressure transfer printing is implemented in a water temperature range of 10 to 40° C.
The printing layer requires enough precision to support photographs, etc., and use of on-demand printing methods, the representative of which is inkjet printing, is becoming popular. Unlike in gravure printing, on-demand printing methods eliminate the need to create plates, and therefore keep the cost low. A method is adopted whereby polyvinyl alcohol resin is used for the printing layer to allow for water-pressure transfer printing (Patent Literature 1).
When a polyvinyl alcohol film is made to float on a water surface and to swell, the surface of the film is wrinkled with fine, wavy lines which will be transferred to the surface of the transfer target to be transfer-printed on, creating an undesirable appearance. To address this problem, a method is adopted whereby starch or other polysaccharide or inorganic matter is used as a filler (Patent Literature 2). However, the filler causes the printing layer to become irregular and consequently affects the sharpness of the resulting print images, which is not desirable.
Another point is that, especially in the case of on-demand printing applications, the printed printing layer must be separated from the base layer. Patent Literature 3 adopts a method whereby embossing or matte finish is applied to the base film for greater ease of handling (Patent Literature 3). This improves machinability, but the fine surface irregularities created by embossing or matting affect the surface of the printing layer, which presents a problem that the precision of print images may drop.
Furthermore, one characteristic of the water-pressure transfer printing films used with these methods is that the side of the film opposite the printing side also comes in direct contact with water and thus wrinkles easily. In addition, water flows that occur when the transfer target is pressed onto a water surface cause the film to wrinkle easily and consequently make it difficult to perform reliable water-pressure transfer printing onto the target surface.